An approximation to turbulent flow that utilizes empirical diagnostic relationships for the mean variables such as wind speed and temperature.

Because these are diagnostic equations rather than prognostic, there are no forecast equations for turbulence statistics of any statistical order including the first-order mean values. An example is the logarithmic wind profile in the surface layer, a similarity theory that gives the wind speed as a function of height without using the equations of motion. See closure assumptions, first-order closure, higher-order closure, nonlocal closure.